1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor (TFT) formed by using a crystallized semiconductor for a semiconductor layer including, a channel forming region, a source region and a source region, particularly, a method for manufacturing a semiconductor having a good crystal characteristic by laser beam radiation.
2. Description of the Related Art
An active matrix type of liquid crystal display device capable of highly fine display has been manufactured so popularly. In the active matrix type of liquid crystal display device, a TFT is provided in each pixel in a pixel portion as a switching element for driving a liquid crystal. The TFT provided in the pixel portion is switched between on and off to change an orientation of the liquid crystal in order to carry out display.
Especially in a crystallized semiconductor film having high electric field effect mobility (typically, a poly-silicon film) of all others, a carrier moves so fast that, when such crystallized semiconductor film is used for a semiconductor layer including the channel forming region, a source region and a source region, it is possible to provide on a substrate same as the pixel portion a drive circuit capable of corresponding to writing of image data even having high resolution, the drive circuit being required to operate at high speed. Such crystallized semiconductor is on the way to practical use.
Demand of a market, however, does nothing but increase for further fineness, higher brightness and lower cost of a liquid crystal display device. In order to solve a cost problem, it is necessary to develop a technology in which a changeover from a quartz plate, which is expensive per substrate, and thereby, raises a price of a liquid crystal display device as a final product, to a cheap plate (a glass plate, for example) and a changeover from a high temperature process, which needs a manufacturing cost such as electric power, to a low temperature process are possible.
Therefore, a laser beam radiation method or a crystallization method using a catalyst element is used as a method for providing a good element at a cheap price and obtaining a good semiconductor film.
Radiation of a semiconductor film with a laser beam enables the semiconductor film to be crystallized or improved in the crystal character. It is because the semiconductor film is fused due to energy of a laser beam and forms innumerable nucleuses so that respective nucleuses would grow mainly in a direction parallel to a film surface of the semiconductor film to form a crystal particle and be solidified.
In growth of such crystal particle after the laser beam radiation, formed on a semiconductor film a convex portion having the height almost equal to the thickness of the semiconductor film due to a collision between adjacent crystal particles.
In the case that a TFT is manufactured by using semiconductor film, which is obtained in the laser beam radiation process under such condition that a convex portion is formed on a surface thereof and on which the formed convex portion remains, as a semiconductor layer including a channel forming region, a source region and a source region, the roughness of the surface of the semiconductor film is reflected in a gate insulating film and a gate electrode, which are later formed on the semiconductor layer, so that it would cause a problem of dispersion of an element characteristic.
Further, there are problems that leakage easily occurs in an OFF operation of a TFT (a drain current flowing in the OFF operation of a TFT becomes high) and that electrostatic focusing occurs to raise an OFF current, since the film thickness of a semiconductor layer is thick at the convex portion. In addition to the above, the roughness of an interface between the semiconductor layer and the gate insulating film traps a carrier (electron) flowing through the channel forming region so that the carrier would become a fixed electric charge to vary a threshold voltage, which causes decline in reliability.
On the other hand, a semiconductor film having high electric field effect mobility can be obtained in the laser beam radiation process under such condition that a convex portion is formed as described above. There is, accordingly, an antinomy relation.